The present invention relates to a method for controlling a treatment unit for the treatment of at least one feed gas, by pressure swing adsorption, in which the treatment unit, commonly known as PSA unit, comprises N adsorption units, N being greater than or equal to 1, operating on a parametrized cycle typically split uniformly into at most N phase times. In the conventional way, the term “phase time” is used to describe the quotient of the duration of the cycle to the number of adsorbers in service.
PSA units are used to produce hydrogen, carbon monoxide, carbon dioxide, to dry gas, to separate the constituents of air with the production of nitrogen and/or oxygen, for carbon dioxide deballasting, etc.
The pressures indicated hereinbelow are all in bar absolute.
In general, the adsorbers of a PSA unit follow, staggered over time, an operating cycle, hereinafter known for convenience as the “PSA cycle” which is split uniformly into as many phase times as there are adsorbers in operation, and which is made up of basic steps, including the steps:                of adsorption at practically a high pressure of the cycle;        of cocurrent depressurization, generally from the high pressure of the cycle;        countercurrent depressurization, generally down to the low pressure of the cycle;        elution at practically a low pressure of the cycle; and        repressurization, from the low pressure of the cycle to the high pressure of the cycle.        
Depending on the application, the depressurization and repressurization steps may involve several sub-steps such as balancing the pressure between adsorbers or between adsorber and tank, etc. The presence of any arbitrary one of these steps in the PSA cycle does not in any way alter the scope of the present invention.
Hereinafter, we shall be concerned with the operation of a PSA unit in the steady state, that is to say outside of transient periods of starting up or shutting down the unit, that generally correspond to special cycles devised for that purpose.
The main constraint on the operation of a PSA unit in the steady state is the level of purity of the gas produced. Under this constraint, the operation of the PSA is therefore generally optimized either to maximize the extraction efficiency (amount of gas produced/amount of this gas present in the feed gas) or to minimize the power consumption, or alternatively to maximize the volume of gas produced.
For this, use is made of a PSA unit control unit designed to modify the parameters of the operating cycle of this unit. Conventionally, it has been proposed for this control unit to permanently receive signals typically representative of the flow rate of the feed gas stream and/or of the flow rate of the produced gas stream.
FIG. 1 of the attached drawings, which illustrates the prior art, depicts a PSA unit 1 for producing hydrogen and a control unit 2. The feed line 3 supplying the gas that is to be treated, is provided with a flow meter 4, the measurements of which are constantly transmitted to the control unit 2.
On the basis of the variation in the feed flow rate, the control unit modifies the duration of the phase time of the cycle so that the higher the flow rate, the shorter the phase time, and vice versa. This control is commonly known as “capacity control”.
Known from other sources is a second type of control which consists in taking account of the purity of the treated gas in order to correct certain parameters of the PSA unit operating cycle. The PSA unit 1 of FIG. 1 for this purpose comprises an apparatus 6 for measuring the hydrogen content of the gas produced by the PSA unit. The measurements from this apparatus are transmitted periodically or continuously to the control unit to control the operation of the PSA unit. In the case, for example, of a produced hydrogen specification of 99.9%, that is to say for a minimum acceptable content of 99.9%, a measurement showing a hydrogen content equal to 99.99% leads to the control unit increasing the phase time, whereas a measurement equal to 99.91% leads to a reduction in this phase time, in order to have a margin at safety. This type of control is commonly known as “purity control monitoring”. In some cases, only the purity control monitoring is employed, but controlling the PSA unit is then tricky.
Side by side with these main forms of control, there may be a certain number of controls internal to the PSA unit, which mean that the pressure cycle is run under the most uniform possible conditions. By way of example, the repressurization flow rate may be kept constant for the duration of the step, by action on a control valve.
However, in a certain number of cases, the main and internal controls sometimes prove inadequate to prevent the production run from becoming polluted.
In particular, in the case of treatment units comprising several adsorbers all in different states at the same instant, it is almost impossible for the parameters of the PSA cycle to be changed instantly and to a significant extent following, for example, a sharp increase in the feed gas flow rate. Amongst other things, it is of course necessary to complete the repressurizing of an adsorber before moving it on to an adsorption phase.
The same is true when the content of an impurity increases sharply in the feed gas, particularly if the impurity concerned is one that is difficult to stop, such as nitrogen or argon, and particularly if this impurity is in a relatively small quantity. By way of example, a nitrogen content increasing from 50 to 500 ppm will not modify the duration of the adsorption phase because its effect on the measurement of the flow rate will be negligible, but it will gradually pollute the adsorbent. The purity control monitoring will react, but with a delay and, depending on the parameters chosen for this control, there will be either a temporary pollution of the production, or the cycle will suddenly go out of control with the consequence of a significant loss of efficiency over some period of time.
One means used to alleviate this disadvantage is that of installing measurement means on the feed gas so as constantly to know its composition, pressure, temperature, density and flow rate characteristics and, on the basis of that, adjusting the cycle, or even changing cycle if the modifications are significant enough to justify so doing, according to the data measured.
FIG. 1 of the appended drawings depicts an analyser 8 for constantly, or sufficiently frequently, determining the composition of the feed gas. This information transmitted to the control unit 2 makes it possible, via the molecular mass of the feed gas which mass is calculated from this analysis, to determine the exact flow rate of feed gas.
Knowledge of the composition and of the flow rate then allows the system to recalculate the optimum cycle parameters.
If the feed gas consists of a mixture of several gases, flow meters and, if necessary, local analysers and/or densimeters can be installed in order, by summing, to reconstruct the composition and the flow rate of the whole feed gas.
The disadvantage with such a system is that it is expensive in terms of hardware (analysers), in terms of operating costs (calibration gas) and in terms of maintenance (calibration, etc). In addition, erroneous analysis due, for example, to an excessive drift in the apparatus, will lead to a cycle not suited to the actual feed gas, and leading either to a loss of production or to pollution.
The object of the present invention is to propose a control method for controlling a PSA unit that is simplified, of negligible cost by comparison with the analysis systems mentioned hereinabove, and which in many cases makes it possible to limit the risk of pollution and/or loss of production when there is a sudden and/or significant variation in the composition of the feed gas fed into this unit.